Method of inspecting a circuit pattern and inspecting instrument

ABSTRACT

A circuit pattern inspecting instrument includes an electron-optical system for irradiating an electron beam on a sample, an electron beam deflector, a detector for detecting secondary charged particles from the sample, and a mode setting unit for switching between a first mode and a second mode. An electron beam current is larger in the first mode than in the second mode, and an electron beam scanning speed is higher in the first mode than in the second mode. The circuit pattern inspecting instrument is configured so that first the sample is observed in the first mode, then a particular position on the sample is selected based on image data produced by an output of the detector in the first mode, and then the particular position on the sample is observed in the second mode.

BACKGROUND OF THE INVENTION

The present invention relates to a circuit pattern inspecting instrumentand a circuit pattern inspecting method, and in particular to aninstrument and a method for inspecting circuit patterns on a wafer orthe like in a semiconductor device manufacturing process.

For a comparison testing method of detecting defects in circuit patternsformed on a wafer in a semiconductor device manufacturing process, therehas been put to practical use an instrument for inspecting the wafers bycomparing images of two or more LSI circuits of the same pattern formedon a single wafer with each other.

Particularly, instruments for pattern comparison and pattern inspectionusing an electron beam are described in Japanese Patent Application LaidOpen No. Sho 59-192943; P. Sandland et al., “An electron-beam inspectionsystem for x-ray mask production”, Journal of Vacuum Science &Technology B, Vol. 9, No. 6, November/December 1991, pp. 3005-3009; D.Fleming et al., “Prospects for x-ray lithography”, Journal of VacuumScience & Technology B, Vol. 10, No. 6, November/December 1992, pp.2511-2515; D. Hendricks et al., “Characterzation of a New AutomatedElectron-Beam Wafer Inspection System, Integrated Circuit Metrology,Inspection, and Process Control IX, February 20-22, 1995, Santa Clara,CA, Proceedings of SPIE, Vol. 2439, May 1995, pp. 174-183; and JapanesePatent Application Laid Open No. Sho 5-258703. In those instruments, forobtaining a practical throughput, it is necessary to acquire images at avery high speed, and at least 100 times (at least 10 nA) the electronbeam current with an ordinary scanning electron microscope is used toensure a sufficient S/N ratio of the images acquired at high speed aswell as a practical inspection speed. The electron beam diameter isspread fairly wider than that in an ordinary scanning electronmicroscope and is about 0.05 to 0.2 μm. This is because of an increasein chromatic aberration caused by widening of the electron energy widthwhich is attributable to a large beam current, a limitation tobrightness of an electron gun and a limitation by the Coulomb effect.

The images formed by such an electron optic system are fed to an imageprocessing unit, in which images of the adjacent circuits of the samepattern are compared with each other for inspection. If a portion havingdifferent brightness occurs between the compared images, the portion isregarded as a defect and coordinates of the portion are stored.

With the above configuration, it is possible to detect even a defect assmall as 0.1 μm or so.

Further, instruments for inspecting defects in semiconductors byreducing the energy of the electron beam with a voltage applied to asample and an electrode disposed close to the sample are disclosed inJapanese Patent Application Laid Open Nos. Hei 7-78855, 9-181139,10-19538, 10-27834, 10-27835, and 11-25901. But these references do notdescribe an instrument having a review function to be described later,or an instrument having a combination of the review function and anenergy analyzing function to be described later.

SUMMARY OF THE INVENTION

Before an inspection is started using one of the above instruments,there are various parameters to be set in advance. As parameters to beset for an electron optic system there are an irradiation energy of anelectron beam, a gain of a signal detection system for image forming.secondary electrons (or charged particles such as back-scatteredelectrons), a pixel size (a minimum picture unit), and the amount of abeam current. On the other hand, it is necessary to set a thresholdvalue for judging whether a signal indicates a defect in comparison oftwo images obtained from the two adjacent areas of the same pattern byan image processing unit. If this threshold value is set too low, thedefect detection sensitivity is high, but it increases a possibilitythat a faultless portion is judged defective. On the other hand, if thethreshold value is set too high, the detection sensitivity becomes toolow.

Optimum values of the above parameters differ depending on a process tobe inspected, a pattern size, and a type of defects to be inspected.Therefore, it is necessary to optimize the parameters by conducting atest inspection in which an image at the coordinates of a detecteddefect is displayed to confirm that a defect desired to be detected hasbeen detected, before a regular inspection.

It is also necessary to for an operator to obtain the image at thecoordinates of the defect after the inspection and check what kind ofdefects has been detected.

In addition to acquiring an image at a high speed so as to see whether adefect is present and then detecting a defect by processing the image,it is also essential to produce an image of a specific small area andobserve the image visually as in the case with an ordinary scanningelectron microscope.

A mode for this observation will be hereinafter referred to as “a reviewmode.” in this specification.

If it is necessary to make distinction between this review and theinspection mode based on high-speed acquisition of images for detectingthe presence of a defect over a relatively large area, the inspectionmode based on high-speed acquisition of images will be referred to as “adefect detecting inspection.”

For the review, it is not necessary to form images at such a high speedas in the defect detecting inspection, but a high resolution image isneeded because it is necessary, not only to recognize whether a defectis present or not, but also to recognize the shape and type of thedefect to some extent.

In the conventional instruments, however, an electron optic system usedtherein is designed so as to be best suited for the acquisition of animage by high-speed scanning at a large current, and it has so far beenimpossible to obtain a resolution high enough for images for the review.Consequently, it was impossible to judge accurately whether a detecteddefect is a true defect or a false defect due to an erroneous detectioncaused by inappropriate setting of parameters. Accordingly, inspectionhas often been conducted with the parameters being not set to optimumvalues.

It is an object of the present invention to provide an inspectinginstrument making possible efficient setting of the conditions forinspecting with an electron beam, defects in repeating design patterns,foreign matters, residues and the like in a semiconductor device on awafer in a semiconductor device manufacturing process, for example.

According to the present invention, the above-mentioned object isachieved by the following configurations.

According to an aspect of the present invention, a circuit patterninspecting instrument includes a cathode for emitting an electron beam;a stage for mounting a sample thereon; an electron-optical system forfocusing the electron beam; a deflector for scanning the electron beamon the sample; a detector for detecting secondary charged particles fromthe sample irradiated by the electron beam; and a mode setting unit forswitching between a first mode and a second mode; wherein in the firstmode, a current of the electron beam is set to a first value and theelectron beam is scanned at a first speed; wherein in the second mode,the current of the electron beam is set to a second value smaller thanthe first value and the electron beam is scanned at a second speed lowerthan the first speed; and wherein the circuit pattern inspectinginstrument is configured so that first the sample is observed in thefirst mode, then a particular position on the sample is selected basedon image data produced by an output of the detector in the first mode,and then the particular position on the sample is observed in the secondmode.

According to another aspect of the present invention, a circuit patterninspecting instrument includes a first electron-optical system includinga first cathode for emitting a first electron beam, a first objectivelens having a first focal length for focusing the first electron beam ona sample positioned at a first sample position, and a first scanningdeflector for scanning the first electron beam on the sample positionedat the first sample position; a first detector for detecting secondarycharged particles generated from the sample positioned at the firstsample position; a second electron-optical system including a secondcathode for emitting a second electron beam, a second objective lenshaving a second focal length shorter than the first focal length forfocusing the second electron beam on a sample positioned at a secondsample position, and a second scanning deflector for scanning the secondelectron beam on the sample positioned at the second sample position;and a second detector for detecting secondary charged particlesgenerated from the sample positioned at the second sample position; animage-forming device for imaging the sample positioned at the firstsample position based on an output of the first detector, and forimaging the sample positioned at the second sample position based on anoutput of the second detector; and a stage for moving a sample betweenthe first sample position and the second sample position; wherein thefirst electron-optical system, the second electron-optical system, thefirst detector, the second detector, and the stage are housed in asingle vacuum chamber; and wherein the circuit pattern inspectinginstrument is configured so that first the sample is observed at thefirst sample position with a current of the first electron beam beingset to a first value and the first electron beam being scanned at afirst speed, then a particular position on the sample is selected basedon image data produced by an output of the first detector, then theparticular position on the sample is moved to the second sample positionby moving the stage, and then the particular position on the sample isobserved by enlarging the particular position on the sample using thesecond electron-optical system with a current of the second electronbeam being set to a second value smaller than the first value and thesecond electron beam being scanned at a second speed slower than thefirst speed.

According to another aspect of the present invention, there is provideda method of inspecting a circuit pattern includes the steps of (a)detecting, using a frist detector disposed at a first position,secondary charged particles from a sample mounted on a stage andirradiated by a first electron beam scanning the sample at a firstscanning speed with a current of the first electron beam being set to afirst value; and (b) detecting, using a second detector disposed at asecond position different from the first position, secondary chargedparticles from a particular position on the sample irradiated by asecond electron beam scanning the sample at a second scanning speedlower than the first scanning speed with a current of the secondelectron beam being set to a second value lower than the first value,the particular position being selected based on an output of the firstdetector.

According to another aspect of the present invention, a method ofinspecting a circuit pattern includes the steps of (a) providing anelectron-optical system for irradiating and scanning a sample having acircuit pattern thereon by a focused electron beam, a detector fordetecting back-scattered electrons or secondary electrons from anelectron beam-irradiated portion of the sample, an image forming unitfor forming an image of the sample based on a detected signal from thedetector, and a difference detecting circuit for comparing an imagesignal obtained by the image forming unit with a reference image signaland thereby detecting a difference between the two image signals; (b)amplifying an output from the detector using an amplifier having a firstamplification factor, the output being obtained by, scanning arelatively large region of the sample with the electron beam of arelatively large electric current at a relatively high-speed; (c) thensupplying the thus-amplified output to the image forming unit to form animage signal; (d) comparing the image signal with a similar image signalobtained from another region of the sample, so as to detect a differencebetween the image signals; (e) determining coordinates of a positionwhere the difference has occurred; (f) scanning a region of a smallerarea than the relatively large region, including the position ofoccurence of the difference, with the electron beam of a smallerelectric current than the relatively large electric current and at alower speed than the relatively high speed; (g) then supplying theresulting output from the detector to the image forming unit via acircuit provided with an amplifier which amplifies the output at alarger amplification factor than the first amplification factor and isprovided with a high-frequency component cut-off filter, to form animage signal; and (h) observing the difference-generating position.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, in which like reference numerals designatesimilar components throughout the figures and in which:

FIG. 1 is a diagram showing an example of an instrument configurationaccording to Embodiment 1 of the present invention;

FIG. 2 is a diagram showing a modification of the instrumentconfiguration of Embodiment 1;

FIG. 3 is a diagram showing an example of an inspection flow in thepresent invention;

FIG. 4 is a diagram showing another example of an inspection flow in thepresent invention;

FIG. 5 is a diagram showing conditions for an inspection and a review;

FIG. 6 is a diagram explaining part of Embodiment 1;

FIG. 7 is a diagram illustrating an operating principle of Embodiment 1;

FIG. 8 is a diagram also illustrating the operating principle ofEmbodiment 1;

FIG. 9 is a diagram showing an example of the operation principle ofEmbodiment 1;

FIG. 10 is a diagram explaining the effect of Embodiment 1;

FIG. 11 is a diagram illustrating a configuration according toEmbodiment 2 of the present invention;

FIG. 12 is a diagram illustrating a configuration according toEmbodiment 3 of the present invention;

FIG. 13 is a diagram illustrating a configuration according toEmbodiment 4 of the present invention;

FIG. 14 is a diagram illustrating a configuration according toEmbodiment 5 of the present invention;

FIG. 15 is a diagram illustrating a configuration of a detection circuitused in Embodiment 1;

FIGS. 16(a) and 16(b) are diagrams illustrating a signal waveform in adefect detecting inspection and a signal waveform in a review,respectively, in Embodiment 1;

FIG. 17 is a diagram illustrating an electron gun unit in electron opticsystems in Embodiments 1 and 2;

FIG. 18 is a diagram illustrating an electron gun unit in the electronoptic systems in Embodiments 1 and 2;

FIG. 19 is a diagram illustrating components of an electron gun used inthe electron optic systems in Embodiments 1 and 2;

FIG. 20 is a diagram illustrating components of the electron gun used inthe electron optic systems in Embodiments 1 and 2;

FIG. 21 is a diagram illustrating the operation of the electron gun usedin the electron optic systems in Embodiments 1 and 2; and

FIG. 22 is a diagram illustrating a concrete configuration of theelectron optic system in Embodiment 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

A first embodiment of the present invention will be described below withreference to FIGS. 1 and 2, which are configuration diagrams of thepresent invention.

An inspection instrument of this embodiment may be divided roughly intoan electron optic system 101, a sample chamber 102, a control unit 104,and an image processing unit 105.

The electron optic system 101 comprises a cathode 1, an electronbeam-extraction electrode 2, a condenser lens 4, a blanking deflector17, a scanning deflector 8, an aperture 5, and an objective lens 7.

Although in FIG. 1 a secondary electron detector 9 is disposed below theobjective lens 7, it may be disposed above the objective lens 7. In thiscase, secondary electrons reach the detector 9 after passing through theobjective lens 7. In the case where the detector 9 is located below theobjective lens 71, a range of a electron beam can be made large in theobjective lens, but a resolving power is limited in a “review” modeoperation described subsequently because of a large aberration of theobjective lens. On the other hand, where the detector 9 is disposedabove the objective lens 7, the resolving power in the review modeoperation can be improved because the aberration of the objective lenscan be made smaller, but the deflection range becomes small. Thus, boththe two arrangements of the secondary electron detector have merits anddemerits. One of the configurations may be selected depending uponapplication of the inspection instrument. A positive high voltage isapplied to the detector 9. A ground level of a preamplifier 12 and thatof a detection circuit 30 both connected to the detector 9 rides on thepositive high potential applied to the detector.

An output signal from the secondary electron detector 9 is amplified bythe preamplifier 12 and is converted into a digital data signal by thedetection circuit 30. This signal is converted into an optical signal byan optical transmitter 34. The optical signal is then received by anoptical receiver 35 which is at the ground potential level, is convertedinto an electrical signal, and then is fed to the image processing unit105.

Although in FIG. 1 the condenser lens 4 is formed by an electromagneticlens, it may be formed by an electrostatic lens as shown in FIG. 2. Forexample, a so-called einzel lens is most suitable for the electrostaticlens. The einzel lens is comprised of three plate electrodes arrangedaxially and two outside ones of the three plate electrodes are at theground potential and a central one of the three is supplied with apositive or negative voltage. But an electrostatic lenses other than theeinzel lens can be used. An electrostatic lens makes possible reductionof the size of the electron optic system.

Returning to FIG. 1, the sample chamber 102 is comprised of a stage 24,an optical height sensor 29, and a stage position measuring unit 31which uses a laser length measuring unit or a linear scale.

The image processing unit 105 is composed of image memory units 18 and19, a calculation unit 20, and a defect recognizing unit 21. An electronbeam image which has been received is displayed on a monitor 22.Instructions and conditions for operations of various components of theinspection instrument are inputted into and outputted from the controlunit 104. Conditions such as an acceleration voltage for electron beamgeneration, a deflection width of the electron beam, a deflection speed,a moving speed of a sample stage, and detector signal pick-up timing areinputted beforehand to the control unit 104. Further, a correctionsignal is generated based upon signals from the optical height sensor 29and the stage position measuring unit 31 and is fed to an object lenspower supply 26 and a scanning signal generator 13.

FIG. 3 is a diagram showing a flow of inspection of an externalappearance of a pattern in a semiconductor device using an electronbeam. With reference to FIG. 3, the following explains a flow ofinspection of an external appearance of a pattern in a semiconductorwafer according to the present invention.

After loading of a wafer, a coarse alignment of the wafer is performedusing an optical microscope. The reason is that an area scanned by anelectron beam in a subsequent operation is so small that there is apossibility that an alignment mark on the wafer positioned mechanicallyin the loading operation is outside the small scanned area. Therefore,the alignment of the wafer by using an optical microscope providing awide viewing field and a low magnification image is performed with anaccuracy of about several tens μm.

Next, conditions for the electron optic system are established. Electronbeam irradiation energy, a pixel size (a minimum image unit), and a beamcurrent are set in accordance with the type of a pattern on a wafer tobe inspected. An optimum electron beam irradiation energy differsdepending on a material forming patterns.

Usually, in the case of an electrically conductive material, a ratherhigh energy of several keV or more is used for attaining highresolution, while in the case of a pattern containing an insulatingmaterial, the energy level is set at 1.5 keV or less for antistaticpurposes.

Next, a pixel size is determined in view of a defect size to bespecially noted. If the pixel size is set excessively small, a timerequired for inspection will increase excessively. Next, an electronbeam current is set. There is a default value of an electron beamcurrent specific to each electron-optical system. But, in the case of awafer which is easily charged electrically, it is preferable that thebeam current is set low, and if the contrast of a defect to be detectedis known to be large beforehand, a high-sensitivity inspection can beperformed by focusing a smaller beam current into a narrower electronbeam.

Then, an electron beam image of the wafer to be inspected is displayedand correction is made for a focal point and astigmatism. This can bedone automatically by picking up the image and executing the correctionon a computer or with use of a dedicated image processing unit.

After completion of the above setting, a fine alignment of the electronbeam image is executed, whereby a coordinate system of the stage andthat of the wafer become coincident with each other exactly. Bymeasuring the coordinates of the stage with a laser length measuringunit for example it becomes possible to inspect a desired position onthe wafer.

Next, an area to be inspected is inputted from a control display, suchas a display of a work station for control.

Then, a test inspection is conducted for judging whether the variousparameters established above are appropriate or not. First, part of apattern to be inspected is designated. This is an area designation inthe test inspection. The test inspection is then carried out. In thetest inspection, images are detected using an electron beam in the samemanner as in the regular defect detecting inspection, and all theacquired images are stored in memory without passing through an imageprocessing circuit.

Next, image processing conditions are established, which conditions arethresholds for judging whether a defect is present or not when twoimages from two adjacent patterns of the same design are compared witheach other in the image processing unit. If the thresholds are set toolow, the defect detecting sensitivity is improved, but it increases apossibility that a faultless portion is judged defective. On the otherhand, if the thresholds are set too high, the detection sensitivitybecomes too low.

In FIG. 3, a threshold of brightness indicates a threshold of adifference between brightness signals of two images. Electron beamimages are very rough (i.e., a low S/N ratio of the image) compared withoptical microscope images. Therefore, even in a case in which twoidentical objects are compared and two images obtained from the twoidentical objects have the same average brightness, a difference inbrightness is present when two corresponding pixels of the two imagesare compared. In view of this, a threshold of a brightness difference isestablished, and only a portion producing a brightness difference abovethe threshold value is detected as a defect when two images from twopatterns of the same design are compared, so that a brightnessdifference between two images caused by roughness produced in the twoimages may not be judged as a defect.

A threshold of an image shift is established so that an error inalignment of two images to be compared may not be judged as a defect.Assuming an image shift below the above-established threshold of animage shift may occur, and only a portion indicating an image shiftabove the threshold of an image shift is judged as a defect.

As to a selection of a filter indicated in FIG. 3, filtering is appliedto an obtained image so as to reduce roughness or enhance edges of thedetected image. An optimum filter is selected by trial and errordepending upon the type of a pattern and defects to be detected.

After the above parameters have been established, image processing isexecuted. The image processing in the test inspection performs acomparison inspection using a series of already acquired images. Thecomparison inspection detects a defect and stores coordinates of thedefect, then displays the position of the defect on the control display.If no defects are detected at this time point, the already establishedthresholds of the image processing parameters are lowered to enhance thedetection sensitivity and the comparison inspection is conducted againwith use of two-dimensional information in a memory.

Next, an image at the coordinates of the defect detected in the testinspection is observed. This is the “review”. Since the image of theportion concerned has already been stored in memory, first the image isdisplayed on a display of the control work station or on a displaydedicated to an image display and a visual observation is made to seewhat kind of defect has been detected. As to a fairly large defect, theshape and type of the defect become clear easily by visual observation.However, if a fine defect is of the size close to a limit size capableof being detected by the inspection instrument or is of a smallcontrast, it is often difficult to judge the defect sufficiently clearlywith visual observation. Then, the conditions of the electron opticsystem are changed over to the conditions of the review so that a highresolution image is produced and it can be easily decided whether thedefect detected in the test inspection is a true defect or a defectintended to be detected. Based upon this result, the image processingparameters are modified and the review is performed again to find outconditions to ensure detection of a defect intended to be detected.

The conventional instrument is not provided with the high-resolutionreview conditions and is designed to form images with a large-currentelectron beam, and therefore, if observation is continued over a longperiod of time and a sample is irradiated with a large quantity ofelectrons, damaging or charging of the sample may occur and make itimpossible to observe the sample. The conventional instrument isintended only for detection of defects and has not a sufficientresolving power for visually judging the details of defects.Consequently, trial and error must be repeated in the review and in thesetting of the image processing parameters, thus requiring a largeamount of time.

According to the present invention, however, because of a good qualityof the review display, it is possible to accurately judge whether theselected image processing parameters are proper or not, and the numberof times of trial and error has decreased remarkably. As a result, thetime of manned operation of the instrument was greatly shortened and anoverall operating efficiency of the instrument was improved.

When the review for setting of the conditions is completed, the defectdetecting inspection is started and an unmanned operation is carried outautomatically. Another review is conducted again upon completion of thedefect detecting inspection. After it has been confirmed by the reviewwhether the detected defect is a true defect or not, the wafer isunloaded. If necessary, the wafer is then transferred to other analyzingand inspecting instruments for various analyses and inspections.

In the case of an automatic inspection of a wafer for which conditionsof defect detecting inspection have already been established, the defectdetecting inspection mode and the review mode are performed according toa flow shown in FIG. 4. More specifically, just after loading of thewafer, the defect detecting inspection is carried out under the existingconditions which have been set, and the review mode is conducted afterthe defect detecting inspection. In the review mode, the detecteddefects are classified depending upon kinds of defects such as foreignmatters, electrical defects, and defective shapes.

The following describes the specifications and features of theconditions of the review and the defect detecting inspection and thedifferences between the conditions of the review and the defectdetecting inspection, which are the main points of the presentinvention.

FIG. 5 shows a comparison in performance between the conditions of thedefect detecting inspection and the review with respect to performance.

The conditions of the defect detecting inspection permit a veryhigh-speed acquisition of images for inspecting the entire wafer surfacein a practical period of time. The time required for forming an imagecorresponding to 100 μm square image with a pixel size of 0.1 μm is atmost 20 msec. For a high-speed acquisition of images, the stage is movedcontinuously and the electron beam is scanned in a direction nearlyperpendicular to the movement of the stage. Further, the instrument isdesigned so as to provide a beam current of at least 20 nA for realizinga high-speed image formation with an S/N ratio sufficient for theinspection. More specifically, the current density obtainable from thecathode was set at least 0.2 mA/sr. A large lens aperture compared withthat of a high-resolution SEM was used and a taking-angle α of anelectron beam from the cathode (i.e., a beam divergence half-angle of aportion of the electron beam emitted from the cathode at a wide anglewhich irradiates the sample) was set at least 6 mrad. The pixel sizeserving as the minimum image unit was set equal to the minimum defectsize desired to be detected. At present, the minimum size of defectswhich pose a problem is approximately 0.05 μm and therefore the pixelsize was set at least 0.05 μm. This value means that the pixel size canbe set larger than 0.05 μm because a larger defect size than 0.05 μm maybe found in some wafers. In some manufacturing processes of wafers to beinspected, there sometimes is a case where the quantity of secondaryelectrons generated is small and a sufficient brightness is not obtainedby a single scanning of an electron beam. In such a case, imagesobtained by scanning an electron beam on the sample in a plural numberof times may be added together. But the smaller the number of times ofthe scanning, the better, because of the problem that the inspectiontime becomes longer. As to the imaging area, or the field of view, thelarger, the better, and therefore it was set at least 50 μm, with 100 μmbeing a standard.

On the other hand, in the review conditions, the current value need notbe large and a current value of 5 nA at most suffices because the imageforming speed is not required to be so high. As to the pixel size, it isrequired to be equal to or less than one-fifth of the defect size forobserving the shape of each defect in detail. Therefore, the pixel sizewas set equal to or less than 0.02 μm. As to the addition of images, itwas set at 2 the nth power, with no upper limit. In the review, forobserving a specific site carefully, the stage is stopped and theelectron beam is scanned in two dimensions. Further, for observing amagnified defect, it is necessary that the imaging area be small, whichis at most 50 μm.

Thus, there is a great difference in considering the pixel size betweenthe inspection and the review mode. In the inspection, even if a probesize is larger than the defect size, it suffices to judge whether adefect is present or not. That is, it suffices for the probe size andpixel size to be equal to or larger than the defect size. If the pixelsize is small, the quantity of electrons irradiated per unit areabecomes smaller and the clock becomes slow due to a long-time radiation,thus giving rise to a problem that the high-speed inspection is notpossible. On the other hand, in the review mode, a sufficient time istaken for acquiring the image of a defect and a probe size of equal toor less than one-fifth of the defect size is required. In the presentinvention, not only the probe size is changed, but also the pixel sizeis changed accordingly between inspection and review.

Now, a more detailed description will be given below about howconditions are changed over between the defect detecting inspection andthe review.

First, the components of the electron optic system and the operationthereof will be described in detail with reference to FIG. 1.

As the cathode 1, a field emission type cathode, particularly adiffusion-supply type thermionic field-emission (Schottky emission)cathode, is preferable, and this electron optic system employed aso-called Zr/O/W type cathode comprising a tungsten tip provided with acoating layer made of zirconium and oxygen thereon. This cathodeprovides stable electron emission over a long time. Further, an angularelectron intensity can be set freely in the range of 0.001 to 1 mA/sr byvarying the extraction voltage. However, with increasing currentdensity, the energy width of emitted electrons also increases, thusleading to an increase of chromatic aberration. An electron beam currentof at least 20 nA was necessary for the inspection, and therefore theangular intensity of electron beams emitted from the cathode 1 is set inthe range of 0.2 to 1 mA/Sr.

The electron beam 6 is extracted from the cathode 1 by applying anappropriate voltage to the extraction electrode 2. The electron beam 6is accelerated by a high negative voltage applied to the cathode 1. Theacceleration voltage could be set at 10 kV or more. This is forsuppressing the aforesaid chromatic aberration caused by an increase inenergy width of the electron beam ascribable to the use of alarge-current electron beam and also for suppressing a phenomenon (theCoulomb effect) in which a large-current electron beam becomes too widedue to mutual repulsion of electrons to be narrowed.

The electron beam 6 travels toward the stage 24 with an energy of about10 kV, then is focused by the condenser lens 4 and is further focusedinto a narrow beam by the objective lens 7, then is irradiated onto asubstrate 10 (a wafer, a chip or the like) to be inspected on the stage24.

The scanned area was set at 50 μm square or more and a distance (anoperational distance) between the objective lens 7 and the substrate 10to be inspected was set at 25 mm in order to ensure a distortion-freeimage even at a peripheral portion. As a result, a focal length of theobjective lens 7 became as long as about 30 mm.

The substrate 10 to be inspected is provided with negative-voltageapplying means for the application of a negative voltage from a highvoltage power supply 25. By adjusting the high voltage power supply 25it becomes easy to adjust the irradiation energy of the electron beamirradiating the substrate 10 at an optimum value. Given that thisvoltage is 9.5 kV and the acceleration voltage. of the electron beam is10 kV, the energy irradiated onto the sample is 500 eV.

An image is obtained by a method in which the electron beam 6 is scannedin only one direction and the stage 24 is moved continuously in adirection perpendicular to the scanning direction.

The detection of signals for image formation is conducted in thefollowing manner. Secondary electrons are generated by the electron beam6 irradiated to the sample 10. Since the secondary electrons isaccelerated rapidly by the voltage applied to the sample 10, it isdifficult to draw in the secondary electrons directly to the detector 9.Therefore, between the substrate 10 and the detector 9 are disposed adeflector formed by a combination of an electric field and a magneticfield, e.g., an ExB deflector 14, and a converter electrode 11 forconverting the accelerated secondary electrons into slow secondaryelectrons.

The ExB deflector 14 is a deflector wherein an electric field and amagnetic field are orthogonal to each other, for the primary electronbeam 6 which enters the ExB deflector 14 from above, a deflecting actioninduced by the magnetic field and a deflecting action-induced by theelectric field are opposite in direction and cancel each other, whilefor the secondary electron beam which enters the ExB deflector 14 frombelow, both such deflecting actions are added together.

After being deflected by the ExB deflector 14, the secondary electronsare irradiated onto the converter electrode 11 and secondary electronsgenerated from the converter electrode are detected by the detector 9.As the detector 9 was used a PIN type semiconductor detector forrealizing a large current (at least 10 nA) high-speed detection. Thesecondary electron signal thus detected is amplified by the preamplifier12, then the amplified signal is subjected to A/D conversion in thedetection circuit 30, and the resulting digital signal is fed to theimage memory unit 18 or 19 which stores data obtained from the secondaryelectron signal and corresponding to a two-dimensional image of thesample. Next, the substrate 10 to be inspected is moved by movement ofthe stage 24, the substrate 10 is irradiated with the electron beam 6emitted from a field emission cathode serving as the cathode 1, and thena secondary electron signal obtained are used to perform a comparisoninspection.

In this way, images of two adjacent semiconductor patterns of the, samedesign spaced several μm from each bother are compared and judged at ahigh speed in the calculation unit 20 or the defect recognizing unit 21,and thereby a defect is detected. The detection of a defect can also beeffected by comparing images of two patterns of the same design on twodifferent chips.

The following describes how to change over to the review mode.

The following are main factors of limiting resolution in the defectdetecting inspection mode: 1) chromatic aberration caused by variationsin energy of electrons in focusing by a lens, 2) the Coulomb effectcaused by mutual repulsion of electrons due to a high current density,3) a finite diameter of an electron source due to the fact that a tipend of the cathode is not a point source. The basics in the change-overto the review mode is such that the electron beam current is madesmaller than that in the defect detecting inspection, therebysuppressing obstacles to focusing the electron beam into a fine beam,such as aberration of the optical system and the Coulomb effect, andthereby forming a high-resolution image with a fine electron beam.Concrete methods will be enumerated below.

A first method is lowering the current density extracted from thecathode. FIG. 6 illustrates a configuration of an electron gun. In FIG.6, a cathode 1 is a so-called Zr/O/W type Schottky emission cathodecomprising a tungsten tip provided with a coating layer thereon made ofzirconium and oxygen. A filament in the cathode 1 is heated by passingan electric current therethrough from a heating power supply 405. A tipend of the cathode projects from a central hole in a suppressorelectrode 403 for suppressing unwanted thermoelectrons.

A potential negative with respect to the cathode 1 is applied to thesuppressor electrode 403 from a Vs power supply 407. The tip end of thecathode 1 is opposed to an extraction electrode 2 and a potentialpositive with respect to the cathode 1 is applied to the extractionelectrode 2 from an extraction power supply 406. The power supplies 405,406 and 407 are floating negative by an amount corresponding to adesired acceleration voltage by means of an acceleration power supply408. As a result, the electron beam extracted from the cathode 1 isaccelerated during its travel to an anode 404 at ground potential. Inthe electron gun thus constructed, the current density extracted can bereduced by reducing an extraction field at the tip end of the cathode.This can be attained by increasing the voltage value of the power supply407 connected to the suppressor electrode 403 or lowering the potentialof the extraction electrode 2.

The following explains the reason why aberration can be reduced byreducing the current density. FIG. 7 shows an angular current intensityof electrons emitted from the cathode and a half-width (hereinafter anenergy width) of energy variations of electrons in the emitted electronbeam. The energy variations in electrons cause chromatic aberration of alens in the electron optic system. Chromatic aberration is proportionalto the energy width as expressed in Equation 1. Thus, if the energywidth is halved, chromatic aberration also is halved. The angularcurrent density for the defect detecting inspection is in a range offrom 0.1 mA/sr to 1 mA/sr, and that in the review mode is in a range offrom 0.001 mA/sr to 0.01 mA/sr. With these conditions, the beam currentbecomes about 100 nA in the defect detecting inspection and about 5 nAor less in the review mode, and it is seen from FIG. 7 that the energywidth becomes one-third and that chromatic aberration also becomesone-third. As a result, the beam diameter also becomes one-third or soand thus the resolution is improved.

dc=Cc·ΔV·α/V ₀  (1)

dc: beam diameter caused by chromatic aberration

ΔV: energy width

Cc: chromatic aberration coefficient

V₀: acceleration voltage

α: beam-divergence half-angle

A second method of improving resolution by reducing an electricalcurrent is replacing the aperture (e.g., the aperture 5 in FIG. 1) withan aperture of a smaller diameter to reduce aberration. As expressed inEquation 1, chromatic aberration of the optical system is proportionalto the beam divergence half-angle α. Therefore, if the diameter of theaperture which determines the beam divergence half-angle is halved,chromatic aberration also is halved. If the beam divergence half-angleis set too small, diffractive aberration in inverse proportion to thebeam divergence half-angle increases. But if the beam divergencehalf-angle is about 1 mrad or more, the diffractive aberration poses noproblem and therefore the beam diameter also becomes nearly half.However, this method requires the use of a mechanical aperture movingmechanism and thus involves problems with reliability and ease of use.

A third method is changing the focal length of a lens so as to changethe magnification of the optical system and thereby reducing the beamdivergence half-angle without moving the aperture. This method issuperior to the second method because the size of the electron source inthe cathode can also be reduced. This will now be described withreference to FIG. 8.

In the defect detecting inspection, an electron beam emitted from acathode is focused to form an image (which is called a crossover) by acondenser lens and then is again focused onto a sample by an objectivelens

On the other hand, in the review mode, the beam taking in half-angle βfrom the cathode is reduced by making the lens action of the condenserlens zero and a beam half-angle β of irradiation by the objective lensalso becomes narrow. As a result, the current value is reduced and thechromatic aberration of the objective lens is reduced to improveresolution. Moreover, the magnification of the entire optical system,which was (b/a)×(d/c) in the defect detecting inspection, becomes assmall as d/(a+b+c) in the review mode, so that the electron source inthe cathode can be reduced in size and hence the resolution is furtherimproved. An electromagnetic lens is used as the condenser lens and itis composed of a coil and a magnetic path. The strength of the lensaction can be controlled by adjusting the value of an electric currentflowing through the coil, so when the lens action is desired to be madezero, the current value may be made zero. Although the description hasbeen directed to the case where the action of the condenser lens is madezero, even if the lens action is not made completely zero, the sameadvantage as above can be expected by adopting such a weak lenscondition that a crossover is not formed.

Also by increasing the strength of the condenser lens in comparison withthat in the defect detecting inspection and thereby shifting theposition of a crossover to a position above the aperture, it is possibleto improve the resolution. This is illustrated in FIG. 9. The effectobtained is the same as in the case of making the lens action of thecondenser lens zero. That is, the magnification of the optical system isexpressed by (b/a)×(d/c), and as will be seen from a comparison betweenFIGS. 8 and 9, b is smaller and c is larger in comparison with those inthe defect detecting inspection, so that the magnification becomessmall. The irradiation angle β can also be made small.

In connection with FIG. 1, there have been explained methods of forminghigh resolution images in the review mode by reducing a beam current,while both the inspection mode and the review mode use a single detectorand a single system of a preamplifier to a processing circuit in the A/Dconverter. On the other hand, as the beam current is reduced, signalsdetected by the detector are reduced considerably. Consequently, if theamplification factor of the detection system in the review mode is thesame as in the defect detecting inspection, an input signal will becomeas small as the minimum bit or less in the, detection circuit 30, withthe result that a satisfactory image is not obtained even if theelectron beam is scanned plural times on a sample and the addition ofobtained images is performed.

In view of this, an additional set of circuits from the preamplifier upto the input to the A/D converter was provided. More specifically, twosystems of signal paths are provided in the detection circuit 30 in FIG.1. This configuration is shown in FIG. 15. In the defect detectinginspection mode, a signal from the detector 9 is inputted directly to anA/D converter 304 via a low-gain amplifier 301. On the other hand, inthe review mode, the signal level from the detector 9 lowers because thebeam current value decreases. The signal path is switched over to ahigh-gain amplifier 302 having an amplification factor higher by theamount corresponding to the lowering of the signal level. Further, thesignal is passed through a high frequency cut-off filter 303 and isinputted to the AD converter 304. The operation of this circuit will bedescribed below with reference to FIGS. 16(a) and 16(b) which areschematic diagrams of signal waveforms in the defect detectinginspection mode and in the review mode, respectively.

As an example, the beam current in the defect detecting inspection modeis set at 100 nA and that in the review mode is set at 500 pA. The gainratio between the low and high-gain amplifiers 301, 302 is set at 200equal to the beam current ratio, and as a result, the average outputvalues of both the amplifiers become equal to each other (100 inarbitrary unit). FIG. 16(a) shows a signal waveform of an image obtainedfrom an area having no pattern on a semiconductor device in the defectdetecting inspection mode, and amplitudes of noise are small because alarge beam current is used.

FIG. 16(b) shows a signal waveform of an image obtained from the samearea in the review mode. Although the average of the signals isapproximately 100, the amplitude of noise is large and the signals swingin a wide range of from −100 to 300 because small signals are amplifiedwith a high gain. If an analog signal swings to a negative value, adigitized value becomes zero. A too large signal exceeds the full scaleof the A/D converter. Thus, there arises a problem that an accuratewaveform is not obtained even if digital signals are added and averaged.

To solve this problem, the high frequency cut-off filter 303 was placedbehind the high-gain amplifier. An input signal to the A/D converter 304has its noise suppressed as indicated by the waveform “FILTERED SIGNAL”in FIG. 16(b). Further, after digitizing this waveform, if the scanningof electron beam is performed plural times and the obtained signals areadded together, there can be obtained an image signal having asatisfactory S/N ratio.

The three methods for improving resolution and the method for solvingthe problem caused by reducing the beam current have been describedabove.

In this embodiment, for the improvement of resolution, the third methodis mainly adopted in which excitation conditions for the condenser lensare changed, and the first method involving changing the current valueextracted from the cathode is adopted as an auxiliary method. Concreteexamples of numerical values will be shown below.

In the defect detecting inspection mode, the angular current intensityof electrons from the cathode was set at 0.5 mA/sr, the magnification ofthe optical system was set at 1.0, and the beam current is set at 100nA. In this conditions, the resolution of the obtained image wasapproximately 0.08 μm. A line profile of an image of a line and spacepattern obtained in the inspection mode is shown in FIG. 10.

In the review mode, the current for the condenser lens was increased andthe magnification was set at 0.2 Further, the angular current intensityof electrons from the cathode was reduced to 0.1 mA/sr by controllingthe high-voltage power supply of the electron gun. As a result, the beamcurrent was decreased to 500 pA and therefore the, gain of thepreamplifier in the detection system was changed to 200 times, employinga filter for reducing a frequency characteristic to about one-tenth. Theelectron beam scanning speed was set at about one-tenth of that in thedefect detecting inspection. Further, the addition of images wasconducted 64 times to compensate for the deterioration of the S/N ratio.At this time, the resolution of the obtained image was about 0.02 μm. Aline profile of the image is superposed in FIG. 10. It is understoodthat the line profile in the review mode rises sharply and is larger inamplitude. This fact clearly shows that the resolution is improved andthat even a fine object can be observed.

Embodiment 2

In Embodiment 1 the same detector was in both the review mode and thedefect detecting inspection mode. In the review mode, however, sincedetected signals are reduced markedly, it is necessary that the gain ofthe preamplifier 12 be increased to 200 to 1000 times in accordance withthe rate of decrease of the electric current. Further, if the gain isincreased while keeping the frequency characteristic as high as in thedefect detecting inspection, there will arise problems that the circuitoscillates and the amplitude of the noise in the circuit becomes largerthan the average of the signals. For this reason it is necessary to adda high frequency cut-off circuit, resulting in complexity of thecircuit. As a result, there is a possibility that noise may beintroduced into detected signals in the defect detecting inspection orthe frequency characteristic may be deteriorated. In this embodiment, tosolve this problem, there is provided a detector used exclusively forthe review mode. FIG. 11 is a configuration diagram of this embodiment.The detector for the review mode is disposed at a position substantiallysymmetric with a detector 9 for the defect detecting inspection withrespect to the optical axis of the electron beam. This detector is oneused in an ordinary type of a scanning electron microscope and iscomposed of a scintillator 52 and a photomultiplier tube 51. Thefrequency response characteristic thereof is DC to 20 MHz.

Secondary electrons emitted from a sample 10 are deflected by an ExBdeflector 14 toward the detector 9 in the defect detecting inspection,while in the review mode they are deflected toward the detectorexclusively for the review. The secondary electrons are irradiated to aconverter electrode opposed to the respective detectors, and thensecondary electrons emitted from the converter electrode are detected.

To a front side of the scintillator 52 is applied a high positivevoltage from a power supply 53, so as to accelerate the secondaryelectrons, collide them with the front side and thereby convert theminto light. The light is multiplied by the photomultiplier tube 51 andis detected.

Change-over of a deflection direction of the secondary electrons isattained by reversing the polarity of a control power supply 31 for theExB deflector 14 (reversing polarities of the applied voltage and thedeflection coil current). Interlocking with this, a signal provided fromone of the detectors is displayed on a monitor 22 via a memory 18. Thischange-over is performed by a control unit 104.

In connection with the construction of Embodiment 2, a more concreteconfiguration of the optical system will be described below withreference to drawings, which employs an electron gun for generating alarge-current electron beam.

First, the configuration of an electron gun optimized for a largecurrent will be explained.

FIG. 17 is a cross-sectional view for explaining an overallconfiguration of an electron gun for a large current and FIG. 18 is aplan view thereof. The electron gun comprises a head unit 301, aremovable coil unit 326, a fixed pole piece 315, a lower pole piece 319,a horizontal adjustment screw 317 of the gun head, a tilt adjustmentscrew 318 of the gun head, a horizontal moving unit 330, a tilt movingunit 331, and an outer heater 320. The head unit 301 of the electron gunis composed of a cathode 304, a suppressor electrode 309, an extractionelectrode 305, a movable pole piece 314, a ceramic insulator 324, and aninner heater 316. The cathode 304, the suppressor electrode 309, theextraction electrode 305, the movable pole piece 314, and the innerheater 316 are suspended from the ceramic insulator 324. The removablecoil unit 326 is made up of a coil 307, a pole piece 323, and a coilfixing piece 327. The coil 307 is fixed to the pole piece 323 with thecoil fixing piece 327. The removable coil unit 326 is configured suchthat it can be removed from the electron gun at the time of baking. Themovable pole piece 314 and the fixed pole piece 315 are spaced from eachother.

The head unit 301 of the electron gun is fixed to the tilt moving unit331, and the tilt moving unit 331 is seated on a curved surface of thehorizontal moving unit 330. The tilt adjustment screw 318 of the gunhead is secured to the horizontal moving unit 330 and the horizontaladjustment screw 317 of the gun head is secured to the pole piece 323 ofthe removable coil unit 326. By pushing the horizontal moving unit 330,the horizontal adjustment screw 317 of the gun head causes the head unit301 of the electron gun to move in the horizontal direction relativelyto the lower pole piece 319. Likewise, by pushing the tilt moving unit331, the tilt adjustment screw 318 of gun head causes the head unit 301to tilt with the cathode 304 as a fulcrum.

The following explains a method of centering the cathode 304 and theextraction electrode 305 in the head unit 301 of the electron gun byreference to FIGS. 19 and which are a cross-sectional view and a planview, respectively, of the head unit of the electron gun. The head unitof the electron gun comprises the cathode 304, suppressor electrode 309,the extraction electrode 305, the movable pole piece 314, the ceramicinsulator 324, the alignment screw 321, the pedestal 328 of thesuppressor electrode, and the pillar 329 for gun heating. The cathode304 is fixed to the pillar 329 for gun heating and the suppressorelectrode 309 is fixed to the pedestal 328 of the suppressor electrode.The extraction electrode 305 and the movable pole piece 314 are madeintegral with the ceramic insulator 324, and alignment screws 321secured to the pedestal 328 of the suppressor electrode fix the ceramicinsulator 324 in place.

Alignment of the extraction electrode 305 with respect to the cathode304 is performed by initially aligning the extraction electrode 305 withthe cathode 304 by pushing the ceramic insulator 324 with the alignmentscrews 321 at the time of mounting the cathode 304 and thereby movingthe movable pole piece 314 and the extraction electrode 305 integralwith the ceramic insulator 324 in parallel with and relatively to thecathode 304, and then fixing the extraction electrode 305 and thecathode 304 together.

A mechanical alignment of the electron gun is effected by a combinationof the operation of aligning the cathode 304 relative to the extractionelectrode 305 and the movable pole piece 314, performed at the time ofmounting the cathode 304 and the operation of aligning the head unit 301of the electron gun relative to an anode electrode 302 and the lowerpole piece 319, performed by extracting an electron beam. Thecombination of the two aligning operations makes possible the moreaccurate alignment of the cathode 304 relative to the electromagneticlens.

It is necessary to secure such a sufficient distance between the movablepole piece 314 and the fixed pole piece 315 to ensure a sufficientwithstand voltage therebetween. However, if this distance is made toolarge, it becomes impossible to generate a strong on-axis magnetic fieldand a stronger excitation is required for retaining the same opticalconditions of the electron gun

Next the withstand voltage between the fixed pole piece 315 and themovable pole piece 314 will be considered.

Suppose that a distance between the fixed pole piece: 315 and themovable pole piece 314 is S (mm), a horizontal shift of the head unit301 of the electron gun is ±1 (mm), and a tilt of the head unit 301 is±100 (mrad). The closest possible distance between the fixed pole piece315 and the movable pole piece 314 after the alignment adjustment isabout (S-1.5) (mm). Suppose a guaranteed dielectric strength in thespace is 5 (kV/mm), then a guaranteed withstand voltage in the spacebetween the movable pole piece 314 and the fixed pole piece 315 is 5(S-1.5) (kV).

FIG. 21 shows an enlarged view of a calculated magnetic fielddistribution obtained with a current of 1 (A) provided to the coil 307of 1000 (T) in the vicinity of the cathode 304 of the above electrongun. FIG. 21 shows magnetic lines of force calculated based upon themovable pole piece 314, the fixed pole piece 315 and the lower polepiece 319, where the inside diameters of the movable pole piece 314 andthe lower pole piece are 14 (mm); and a spacing between the pole piecesis 8 (mm). From this result it is seen that even if the distance betweenthe movable pole piece 314 and the fixed pole piece 315 is 8 (mm), astrong on-axis magnetic field can be produced by magnetic coupling inthe space therebetween.

The small-sized large-current electron gun structure capable of stableoperation has been described above. Next, the following descriptiondescribes the configuration of an optical system employing this electrongun.

The electron optic system shown in FIG. 22 comprises an electron gununit 342, an optics column unit 360, a stage unit 358, an imageprocessing unit 357, a display device 359, a deflection control unit354, a lens control unit 355, a retarding power supply 356, and a powersupply 353 for the electron gun. The electron gun unit 342 is made up ofa removable coil unit 326, a coil 307, a fixed pole piece 315, an anode302, a lower pole piece 319, a movable pole piece 314, an extractionelectrode 305, a suppressor electrode 309, and a cathode 304. Thecathode 304 is configured so as to be provided with an electric currentIf for heating from the power supply 353 for the electron gun and adesired acceleration voltage V0. Further, a reverse bias voltage Vs withrespect to the cathode 304 is applied to the suppressor electrode 309,and a positive bias voltage V1 is applied to the extraction electrode305.

The optics column unit 360 is made up of a movable aperture 343, ablanking plate 344, a Faraday cup 345, a condenser lens 347, asemiconductor detector 349, a detector 371, a deflector 346, anobjective lens 348, an ExB deflector 362, and a converter electrode 363.The semiconductor detector 349 operates at a sampling frequency of 10MHz to 200 MHz and is supplied with a high voltage for attractingsecondary electrons generated from a sample 350 to the semiconductordetector 349. The detector 371 operates at a sampling frequency up toabout 10 MHz and can be supplied with a high voltage like thesemiconductor detector 349. The ExB deflector 362 is configured suchthat the polarities of the electrostatic and electromagnetic deflectorscan be reversed between the inspection mode and the review mode.

The stage unit 358 is made up of a sample 350, a sample holder 351, aceramic insulator 324, a stage driving unit 352, and a sample chamber361. The sample 350 and the sample holder 351 are electrically insulatedfrom the stage driving unit 352 by the ceramic insulator 324, and aresupplied with a retarding voltage Vr.

The cathode 304 has a voltage V0 applied thereto and is supplied with acurrent If for gun heating, the extraction electrode 305 is suppliedwith a positive bias voltage V1 with respect to the cathode 304, and thesuppressor electrode 309 is supplied with a reverse bias voltage Vs,such that an electron beam. 308 is emitted with an energy V0.

In the inspection mode, the electron beam 308 thus emitted is focused bythe magnetic field generated by the removable coil 326 in the electrongun unit 342 to form a crossover between the blanking plates 344. Then,the excitation of the condenser lens 347 is adjusted so that the overallmagnification of the optical system becomes 0.5 to 1.5 times and theelectron beam is focused onto the sample 350 by the objective lens 348.The amount of a probe current of the electron beam 308 is determined byboth the diameter of the movable aperture 343 and the angular currentintensity of the electron beam 308 and it is adjustable in the range offrom 20 (nA) to 200 (nA). The retarding voltage Vr is applied to thesample 350 from the retarding power supply 356, and is varied to adjustthe energy of the incident electron beam 308.

A sawtooth wave of 10 kHz to 200 kHz is generated in the deflectioncontrol unit 354 as a deflection signal and thereby the electron beam308 is deflected by the deflector 346. The deflection control unit 354controls the stage driving unit 352 so as to move the stage 351 in adirection perpendicular to the direction of the electron beam deflectionsuch that the electron beam 308 scans the sample 350 two-dimensionally.Further, the deflection control unit 354 is configured such that avoltage can be applied to the blanking plate 344 so as to intercept theelectron beam 308 by the Faraday cup 345 when the electron beam 308 isnot desired to irradiate the sample 350.

The ExB 362 deflector is adjusted such that only secondary electrons 365generated from the sample 350 are deflected away from the optical axis,then they are irradiated onto the converter electrode 363 and finallyattracted to the semiconductor detector 349. The secondary electrons 365detected by the semiconductor detector 349 are converted into a digitalsignal by an A/D converter 364, and the digital signal is used to forman image by an image processing unit 357.

The image processing unit 357 calculates a difference image between areference image 366 picked up initially and an image 367 picked up forcomparison at a position different from a position of the referenceimage 366 on the wafer and transmits the difference image to the displaydevice 359 as a defect-indicating image 368. Also the image processingunit 357 can transmit the image 367 picked up for comparison andcontaining a defect to the display device 359.

For example, suppose that this SEM type circuit pattern inspectinginstrument inspects a wafer containing a pattern size of 0.1 (μm), thenit is preferable that the probe size is equal to or less than 0.1 (μm)on the sample 350.

In the case of performing the above inspection under the conditions thatthe focal length of the objective lens 348 is 30 to 40 (mm) and theprobe current is 50 (nA) to 150 (nA), it is essential that the totalmagnification of the optical system is in the range of 0.5 to 1.5 forsuppressing chromatic aberration of the objective lens 348, andconsequently, it is necessary to suppress aberration of the electrongun.

In the case of conducting the inspection with a conventional electrongun incapable of handling a large electric current, chromatic aberrationand spherical aberration of the electron gun as defined on an objectside are as large as 45 (nm) to 60 (nm) and 35 (nm) to 50 (nm),respectively, and consequently, the probe size on the sample 350 becomes0.15 (μm) to 0.3 (μm).

When the electron gun according to the present invention is used with anacceleration voltage of 10 (kV) and an excitation of 800 (AT) to 1000(AT), there can be attained a focal length of the electron gun of 8 (mm)to 11 (mm) as defined on the object side, and therefore chromaticaberration and spherical aberration of the electron gun as defined onthe object side become 15 (nm) to 20 (nm) and 1 (nm) to 2 (nm),respectively, and as a result, a probe size of 0.05 (μm) to 0.1 (μm) isattained on the sample 350.

Further, when the electron gun of the invention is used with amagnification of 5 to 10 and the reducing optical system for reductionto 0.1 to 0.5 is formed of the condenser lens 347 and the objective lens348, influences by contaminants adhering to the blanking plate 344, themovable aperture 343, and the Faraday cup 345 are reduced. For example,suppose that an image drift of 0.5 (μm) is caused by contaminants whenall the lenses are used to provide a magnification of 1, adoption of theabove optical conditions reduces the drift to 0.05 (μm) to 0.1 (μm).

On the other hand, for confirming a detected defect on a circuit patternby forming a sharp high-resolution image of the defect, the defect imageis observed with the optical conditions being set for the reviewconditions. In the review conditions, the excitation of the coil 307 isadjusted so that an electron beam 369 forms a crossover above themovable aperture and the probe current becomes 100 (pA) to 5 (nA).Further, the excitation of the condenser lens 347 is adjusted so thatthe overall magnification of the optical system becomes 0.2 to 0.3 andthe electron beam is focused onto the sample 350 through the objectivelens 348. Secondary electrons 370 generated from the sample 350 aredetected by a detector 371 by using the ExB deflector 362 with itspolarity reversed from that for the inspection mode. The detectedsecondary electrons 370 are imaged on the display device 359.

A stable inspection instrument capable of switching between theinspection and review conditions instantly and handling a large electriccurrent is realized by mounting the electron gun of the invention on anSEM type circuit pattern inspecting system provided with the reviewfunction and by adjusting the electron gun as described above.

Embodiment 3

In case of some particular kinds of defects, a contrast of an image ofthe detected defects may disappear if the conditions for the review modehave not been set appropriately. For example, the contrast of an imageof some defects in interconnections by via holes disappear after theirradiation of an electron beam is repeated several times. Therefore thenumber of times of addition of images needs to be limited when suchdefects are observed in the review mode, and as a result, an electronbeam current needs to be made rather large to obtain images of asatisfactory S/N ratio.

For some kinds of defects, in the defect detecting inspection mode usinga large-current electron beam, the stronger the retarding electric field(the electric field applied to a wafer to be inspected for deceleratingthe electron beam), the larger the contrast of the image of defects,but, on the other hand, when a small electron beam current is used, theweaker the retarding electric field, the more enhanced the contrast ofthe image of the defects.

As an example, the following explains a method of setting the conditionsfor the review mode when a defective interconnection is observed.

A semiconductor manufacturing process includes a process step of formingconnection holes and embedding an electrically conductive material intothe connection holes for through connections between laminated upper andlower layers. Defective formation of the connection holes or defectiveembedment of the material causes a complete electrical disconnection ora very high resistance. When the defect detecting inspection isperformed in the process step for producing the interconnections,embedded holes having good electrical conduction appear bright andnon-conductive embedded holes appear dark.

Further, embedded holes having marginal electrical conduction appearintermediate between bright and dark and are detected as defects whenthey are compared with the embedded holes having perfect conduction. Theembedded holes having marginal electrical conduction come to appear asbright as those having perfect conduction after several tens of additionof images are performed with reduced electrical current in the reviewmode, and usually the review does not provide any useful information.

In view of the above fact, in the defect detecting inspection forconnection holes, the brightness level of each detected defect is storedin a memory in combination with coordinates of the defect, and in thesubsequent review mode for the gray defect, the optical conditions areset automatically such that the beam current does not become equal to orless than 5 nA and the number of times of addition of images is limitedto ten. On the other hand, in the review mode for very dark defects, thebeam current is set as low as possible and is limited to 500 pA.

The above is just an example. The point is that defects are classifiedautomatically using information obtained from the images in the defectdetecting inspection such as brightness of the images, defect sizes anddefect shapes and then are automatically correlated. with the suitableoptical conditions for the review mode such as a scanning area, a beamcurrent, a scanning speed, and the number of times of addition ofimages. In the present invention, mechanical movements such as themovement of the aperture are eliminated to the utmost in changing of theoptical conditions, and consequently, the optical conditions can bechanged in an instant depending upon defects.

Embodiment 4

In this embodiment an electron optic system for the review mode isseparated from an electron optic system for the defect detectinginspection. A more detailed description will be given below withreference to FIG. 12, which is a configuration diagram of thisembodiment.

An inspection instrument of this embodiment is roughly divided into anelectron optic system 101 for the defect detecting inspection, anelectron optic system 200 for the review, a sample chamber 102, acontrol unit 104, and an image processing unit 105. The components otherthan the electron optic system 200 for the review are substantially thesame as in Embodiment 1.

The electron optic system 200 for the review is made up of a cathode201, an electron beam extraction electrode a 202, a condenser lens 204,a scanning deflector 208, an aperture 205, and an objective lens 207. Asecondary electron detector 209 is disposed above the objective lens 207and an output signal from the secondary electron. detector 209 isamplified by a preamplifier 212 and is transmitted to a low-speed imagedisplay circuit 218.

A retarding voltage is applied to a stage 24. When the stage is movedfrom below the electronic optic system 101 for the defect detectinginspection to below the electronic optic system for the review, it issometimes preferred, depending on the structure of the stage, that thestage is moved after once disconnecting the stage from the retardingvoltage. Therefore, an appropriate selection may be made in view of theretarding voltage applied and the type of a substrate to be inspected.

The following describes the points of this embodiment which are thespecifications, differences and features of the electron optic systemfor the review and the electron optic system for the defect detectinginspection.

The electron optic system 101 for the defect detecting inspection canacquire images at a very high speed so as to perform the inspection ofthe entire wafer surface in a practical period of time. The timerequired for forming an image of 100 μm square with a pixel size of 0.1μm is equal to or less than 20 msec. The stage is moved continuously forobtaining images at a high speed and the electron beam is scanned in adirection perpendicular to the moving direction of the stage. Theinspection instrument was designed so as to provide a beam current of atleast 20 nA for realizing a high-speed image formation with an S/N ratiosufficient for the inspection. To be more specific, the angular currentintensity obtained from a cathode 1 was set at about 1 mA/sr which is astably-obtainable limit value and is about twenty times as large as thatof a high-resolution SEM. Further, a lens aperture is made largercompared with that in the high-resolution SEM and the beam taking-inangle α for the electron beam emitted from the cathode 1 (i.e., a beamdivergence half-angle of a portion of the electron beam emitted from thecathode 1 at a wide angle which irradiates a sample 10) was set at about20 times larger.

If an image is distorted at its periphery or image resolution at theperiphery of the image is deteriorated compared with that at the centralportion of the image, the inspection sensitivity becomes non-uniform, soit is necessary that the area scanned by the electron beam is more thanenough. For this reason, the focal length and the operational distanceof an objective lens were made considerably longer than those of anordinary type of a high-resolution SEM.

With an increase of the entrance half-angle β of the electron-beamirradiating a sample, the depth of focus becomes smaller. Since theangle β is the angle α divided by the overall magnification M of theentire optical system, the magnification M cannot be made so small andtherefore the magnification M is made fairly larger compared with thatof the high-resolution SEM.

With the above design, a large electric current necessary for a highspeed imaging can be obtained while ensuring the resolution required forthe defect detecting inspection. In, this electron optic system, asnoted above, it is apparent from the magnification of the optical systemand the focal length and the operational distance of the objective lensthat, even if the electron beam current is simply reduced, the sameresolution as that of the high-resolution SEM cannot be obtained.

In the electron optic system for the review, high-speed operation is notso important. Besides, since the coordinates of a area to be observedare already known accurately, the field of view having 20 μm suffices.Therefore, the electron beam current may be set small. Moreover, thefocal length and the operational distance of the objective lens may bemade short and the magnification of the optical system can be set small.From these points it is possible to obtain an image having the samehigh-resolution as in the ordinary high-resolution SEM.

A description will now be given of the electron optic system 200 for thereview. As the cathode 201 there was used a Zr/O/W type cathode which isa diffusion-supply type thermionic field-emission (Schottky emission)cathode similar to that used in the optical system for the defectdetecting inspection. As the angular current intensity is increased, theenergy width of emitted electrons also increases and so does a chromaticaberration. Since the beam current in the electron optic system for thereview may be set equal to or less than 100 pA, the angular currentintensity was set equal to or less than 0.05 mA/Sr. As a result, theenergy width of the electron beam 206 decreases to about one-third toone-fourth of that in the electron optic system for the defect detectinginspection and chromatic aberration so much decreases. The electron beam206 is extracted from the cathode 201 by applying a voltage to theextraction electrode 202. The electron beam 206 can be accelerated byapplying a high negative voltage to the cathode 201. In the opticalsystem 200 for the review, the acceleration voltage can be varied in therange of 500V to 10 kV. In the optical system 200 for the review, sincea small electric current will suffice, chromatic aberration is small asnoted above and the Coulomb effect can be ignored. Thus, since asufficiently small beam diameter can be obtained with a low accelerationvoltage, the acceleration voltage was set at 2 kV as a standard value.

The electron beam 206 travels toward the stage 24 with an energy of 2kV, is focused by the condenser lens 204, then is focused into a finerbeam by the objective lens 207, and is irradiated onto the substrate 10to be inspected (e.g., a wafer or a chip) on the stage 24.

The objective lens 207 is disposed in close proximity to the substrate10 and the operational distance thereof was set at 5 mm. A focal lengthof the objective lens is 8 mm. As a result, there could be realized anobjective lens of small aberration. The resolution depends mostly onchromatic aberration. Thus, in comparison with the objective lens forthe defect detecting inspection, an aberration coefficient is aboutone-fourth, the energy width of the electron beam is about one-third ofthat in the optical system for the defect detecting inspection, andtherefore the resolution is about one-twelfth. The substrate 10 to beinspected is common to both the defect detecting inspection and thereview and therefore a negative voltage can be applied from ahigh-voltage power supply 25. By adjusting the high-voltage power supply25, the irradiation energy of the electron beam onto the substrate 23can be adjusted to an optimum value. Since images of different contrastscan be obtained by changing the irradiation energy, it is possible toobtain various information such as information on whether a detecteddefect is caused by a difference in shape or in material, or by anelectrical conduction.

For formation of images, the electron beam 206 is scannedtwo-dimensionally with the stage 24 being fixed. secondary electronsgenerated by the electron beam 206 irradiated onto to the substrate 10are accelerated by the voltage applied to the substrate 10 as is thecase with the optical system 101 for the defect detecting inspection.

Since the objective lens 207 is disposed as close as below about 30 mmto the substrate 10, the detector 209 is disposed above the objectivelens 207, and the secondary electrons is passed through the center ofthe objective lens 207. Since the voltage applied to the substrate 10 islower as compared with that in the optical system 101 for the defectdetecting inspection, the energy of the secondary electron is small, butit is still difficult to draw in the secondary electrons directly intothe detector 209. Therefore, the secondary electrons are irradiated ontoa converter electrode 211 and the resulting secondary electrons aredetected by the detector 209. As the detector 209 there was used adetector comprising a phosphor and a photomultiplier tube which are usedin the conventional SEM. A secondary electron signal thus detected isamplified by the preamplifier 212 and is A/D-converted by an A/Dconverter 230, then the resulting digital signal is displayed on adisplay monitor 22 for image observation and, if necessary, it is storedas an image file in an external storage 219 such as a disc or can beprinted out.

The numerical values of various functions referred to above inEmbodiment 4 are only examples. What is essential in this embodiment isthat the electron optic system for the review capable of attaining abeam diameter about one-tenth of that in the electron optic system forthe defect detecting inspection is arranged side by side in the samesample chamber for the electronic optic system for the defect detectinginspection and that the defect detecting inspection mode and the reviewmode can be changed over from each other quickly by only a parallelmovement of the stage.

Embodiment 5

In this fifth embodiment, an X-ray detector 240 is installed in a lowermagnetic path of the objective lens 207 in the electron optic system 200for the review used in Embodiment 4 so as to detect characteristicX-rays generated by the irradiation of an electron beam. As a result, itbecame possible to make EDX (Energy-Dispersive X-ray) analysis and hencepossible to identify the material of a defect in the review mode. FIG.13 illustrates the objective lens and the vicinity thereof with anannular X-ray detector incorporated therein, the detector having acentral hole.

Alternatively, the operational distance of the objective lens maybe madesomewhat larger than in Embodiment 1 and an X-ray analyzer may beinserted between the objective lens and the sample.

Embodiment 6

In the inspection using an electron beam, not only a defect of a shapebut also electrical conduction and non-conduction can be inspected. Thisis because a conductive portion is not charged with electricity and onlya non-conductive portion is charged electrically, so that the energy andtrajectories of resulting secondary electrons changes and the brightnessof the image differs.

This results in generation of a particularly large contrast uponirradiation of a large-current electron beam. But in the electron opticsystem for the review, the difference in electric charging is smallbecause of a small electric current in the electron optical system forthe review and a contrast difference is difficult to appear.

In view of this point there was provided a secondary electron energyfilter capable of filtering even a slight difference in the energy ofsecondary electrons with a high sensitivity. The energy filter wasdisposed above the object lens. This analyzer is illustrated in FIG. 14.A semi-spherical mesh 220 is disposed above an objective lens 207 so asto block the trajectories of secondary electrons. Centrally of the mesh220 is formed a hole for passing a primary electron beam 206. A voltageof about ±20V with respect to the potential of the substrate 10 to beinspected is applied to the mesh 220, whereby the state of electriccharging of the substrate can be imaged. The secondary electrons have anenergy distribution peaking at about 2 eV with respect to the potentialφ of the position for emitting secondary electrons. Thus, the energypeak of the secondary electrons with respect to ground potential is(−φ+2) eV.

For example, if a retarding potential of 500V is applied to thesubstrate 10, the peak of the secondary electron energy becomes 502 eV.If a primary electron beam irradiates an area which is made locallynon-conductive by a defect in a pattern and which is otherwiseelectrically connected to the substrate 10, the area is chargednegatively, and consequently, the energy of secondary electrons emittedfrom the area is higher by the amount corresponding to a potentialgenerated by charging. Suppose the potential generated by charging is 5V, then the energy of the secondary electrons is 507 eV. Therefore, if avoltage of −505V is applied to the mesh 220, most of secondary electronsfrom an uncharged area cannot pass the mesh 220, and consequently, onlythe charged area appears in an image. In this way even a slightlycharged area can be detected as a contrast difference.

Although only the case of negative charging has been described here, themechanism of electric charging by the irradiation of an electron beam inan actual semiconductor device is complicated and there sometimes occursa positive charging. In such a case, the voltage applied to the mesh 220may be made equal to or somewhat positive with respect to the retardingpotential. In this case, a positively charged area appears dark and theremaining area appears bright.

Resolution of about 0.1V can be obtained by the energy analyzer of thisembodiment. In the review, therefore, even if the amount of charging ata defective portion is small due to a small electron beam current, it ispossible to, easily identify the defect because the contrast produced bya potential is enhanced.

The present invention configured as above provides the followingadvantages.

Acquisition of an image signal with a sufficient S/N ratio and efficientsetting of the inspection conditions are realized by providing adetection circuit with two processing circuit paths independentlyoptimized for the defect detecting inspection by the high-speedacquisition of images for detecting the presence of defects over arelatively wide area and the review by observing visually an image of aspecific narrow portion detected by the defect detecting inspection,respectively. As a result, not only speed-up of the inspection processcould be attained but also the results of the inspection became morereliable.

Moreover, a first electron optic system for the defect detectinginspection and a second electron optic system exclusive for the reviewby observing a specific narrow portion detected by the defect detectinginspection are disposed side by side within a single vacuum vessel andthe defect detecting inspection mode and the review mode can be changedover from one another by a mere movement of a sample-carrying stage, andconsequently this combination makes possible a quick and highly reliableinspection.

Also, the inspection system of the present invention is provided with afirst detector for detecting the presence of a defect, a second detectorexclusively for review by observing a specific narrow portion detectedby the first detector, and a deflector circuit which directsback-scattered electrons or secondary electrons generated from a sampleirradiated by an electron beam to the first detector in the defectdetecting inspection and directs the back-scattered electrons orsecondary electrons to the second detector in the review, andconsequently, this inspection system makes possible a highly reliableinspection using a signal with reduced noise and reduced deteriorationof its high frequency characteristics.

What is claimed is:
 1. A circuit pattern inspecting instrumentcomprising: a cathode for emitting an electron beam; a stage formounting a sample thereon; an electron-optical system for focusing theelectron beam; a deflector for scanning the electron beam on the sample;a detector for detecting secondary charged particles from the sampleirradiated by the electron beam; and a mode setting unit for switchingbetween a first mode and a second mode; wherein in the first mode, acurrent of the electron beam is set to a first value and the electronbeam is scanned at a first speed; wherein in the second mode, thecurrent of the electron beam is set to a second value smaller than thefirst value and the electron beam is scanned at a second speed lowerthan the first speed; and wherein the circuit pattern inspectinginstrument is configured so that first the sample is observed in thefirst mode, then a particular position on the sample is selected basedon image data produced by an output of the detector in the first mode,and then the particular position on the sample is observed in the secondmode.
 2. A circuit pattern inspecting instrument according to claim 1,wherein an area scanned by the electron beam in the second mode issmaller than an area scanned by the electron beam in the first mode. 3.A circuit pattern inspecting instrument according to claim 1, wherein animage in the second mode is formed using a size of a pixel serving as aminimum picture unit which is smaller than a size of a pixel in thefirst mode.
 4. A circuit pattern inspecting instrument according toclaim 1, wherein an electron beam spot on the sample produced by theelectron beam in the second mode is smaller than an electron beam spoton the sample produced by the electron beam in the first mode.
 5. Acircuit pattern inspecting instrument according to claim 1, wherein theelectron-optical system includes an objective lens located closest tothe sample, and a second lens disposed on a cathode side of theobjective lens; and wherein in the second mode, the second lens isadjusted to provide a small magnification of the electron-opticalsystem.
 6. A circuit pattern inspecting instrument according to claim 1,wherein the electron-optical system includes a diaphragm having aplurality of apertures having different diameters; wherein one of theapertures is used in the first mode; wherein another one of theapertures is used in the second mode; and wherein the diameter of theaperture used in the second mode is smaller than the diameter of theaperture used in the first mode.
 7. A circuit pattern inspectinginstrument according to claim 5, wherein the second lens is a condenserlens; and wherein in the second mode, a position of a crossover formedby the electron beam is shifted.
 8. A circuit pattern inspectinginstrument according to claim 1, wherein in the second mode, a length ofa side of an area scanned by the electron beam is reduced to half orless of a side of an area scanned by the electron beam in the firstmode.
 9. A circuit pattern inspecting instrument according to claim 1,further comprising: an image-forming circuit for forming an image byadding image signals obtained by scanning the electron beam a pluralityof times; an image-difference detection circuit for detecting an imagedifference between a reference image and an image obtained in the firstmode; and a circuit for changing, in the second mode, a magnification ofthe electron-optical system, a current of the electron beam, a scanningspeed of the electron beam, and a number of times of adding imagesignals in the image-forming circuit in accordance with an outputobtained in the first mode from the image-difference detection circuit.10. A circuit pattern inspecting instrument comprising: a firstelectron-optical system including a first cathode for emitting a firstelectron beam, a first objective lens having a first focal length forfocusing the first electron beam on a sample positioned at a firstsample position, and a first scanning deflector for scanning the firstelectron beam on the sample positioned at the first sample position; afirst detector for detecting secondary charged particles generated fromthe sample positioned at the first sample position; a secondelectron-optical system including a second cathode for emitting a secondelectron beam, a second objective lens having a second focal lengthshorter than the first focal length for focusing the second electronbeam on a sample positioned at a second sample position, and a secondscanning deflector for scanning the second electron beam on the samplepositioned at the second sample position; a second detector fordetecting secondary charged particles generated from the samplepositioned at the second sample position; an image-forming device forimaging the sample positioned at the first sample position based on anoutput of the first detector, and for imaging the sample positioned atthe second sample position based on an output of the second detector;and a stage for moving a sample between the first sample position andthe second sample position; wherein the first electron-optical system,the second electron-optical system, the first detector, the seconddetector, and the stage are housed in a single vacuum chamber; andwherein the circuit pattern inspecting instrument is configured so thatfirst the sample is observed at the first sample position with a currentof the first electron beam being set to a first value and the firstelectron beam being scanned at a first speed, then a particular positionon the sample is selected based on image data produced by an output ofthe first detector, then the particular position on the sample is movedto the second sample position by moving the stage, and then theparticular position on the sample is observed by enlarging theparticular position on the sample using the second electron-opticalsystem with a current of the second electron beam being set to a secondvalue smaller than the first value and the second electron beam beingscanned at a second speed slower than the first speed.
 11. A circuitpattern inspecting instrument according to claim 10, wherein the firstelectron beam irradiates a wider area on the sample than does the secondelectron beam.
 12. A method of inspecting a circuit pattern comprisingthe steps of: (a) detecting, using a first detector disposed at a firstposition, secondary charged particles from a sample mounted on a stageand irradiated by a first electron beam scanning the sample at a firstscanning speed with a current of the first electron beam being set to afirst value; and (b) detecting, using a second detector disposed at asecond position different from the first position, secondary chargedparticles from a particular position on the sample irradiated by asecond electron beam scanning the sample at a second scanning speedlower than the first scanning speed with a current of the secondelectron beam being set to a second value lower than the first value,the particular position being selected based on an output of the firstdetector.
 13. A method of inspecting a circuit pattern comprising thesteps of: (a) providing an electron-optical system for irradiating andscanning a sample having a circuit pattern thereon by a focused electronbeam, a detector for detecting back-scattered electrons or secondaryelectrons from an electron beam-irradiated portion of the sample, animage forming unit for forming an image of the sample based on adetected signal from the detector, and a difference detecting circuitfor comparing an image signal obtained by the image forming unit with areference image signal and thereby detecting a difference between thetwo image signals; (b) amplifying an output from the detector using anamplifier having a first amplification factor, the output being obtainedby scanning a relatively large region of the sample with the electronbeam of a relatively large electric current at a relatively high speed;(c) then supplying the thus-amplified output to the image forming unitto form an image signal; (d) comparing the image signal with a similarimage signal obtained from another region of the sample, so as to detecta difference between the image signals; (e) determining coordinates of aposition where the difference has occurred; (f) scanning a region of asmaller area than the relatively large region, including the position ofoccurrence of the difference, with the electron beam of a smallerelectric current than the relatively large electric current and at alower speed than the relatively high speed; (g) then supplying theresulting output from the detector to the image forming unit via acircuit provided with an amplifier which amplifies the output at alarger amplification factor than the first amplification factor and isprovided with a high-frequency component cut-off filter, to form animage signal; and (h) observing the difference-generating position. 14.A method of inspecting a circuit pattern according to claim 12, whereinin the step (a), the stage is moved continuously in a first direction,and the first electron beam scans the sample in a second directionperpendicular to the first direction; and wherein in the step (b), thestage is held stationary.